Sputtering (also known as vacuum sputtering or physical vapor deposition or PVD) is uniquely suited for controlled and reproducible deposition of thin films on an atom layer by atom layer basis. Sputtering is the preferred method for adding the essential magnetic layer to memory disks used in computer disk drives. Sputtering is also used extensively in semiconductor device fabrication. In addition to depositing thin layers of metals, sputtering is also a means for adding thin layers of insulating materials such as aluminum oxide, an important step in the fabrication of write/read heads for disk drives.
Details of the mechanism of sputtering may be found in texts on plasma physics such as “Glow Discharge Processes” by B. Chapman (John Wiley and Sons, NY 1980, ISBN 0-471-07828-X, Chapter 6, pages 177-194). Briefly, sputtering takes place in a vacuum chamber. A small amount of inert argon gas added to the vacuum becomes ionized in an electrical field applied between two electrodes. The electrically accelerated argon atoms impact the source target electrode and knock loose atoms. The loosened target atoms are ejected and transfer through the vacuum space and build up a desired layer on an opposing electrode surface.
The physical arrangement of a target and its backing plate is illustrated in FIGS. 1A, 1B, 1C and 1D for circular targets of the type designed for the widely-used Perkin-Elmer type 2400 and 4400 sputter systems. However, the key features are also common to target assemblies for other systems.
Typically, the material of the source target (11) is a 3 to 6 mm thick layer (FIG. 1A) which is bonded to a reusable copper backing plate, (12), FIG. 1B, by a low melting solder layer (13), shown enlarged for clarity in FIG. 1C. In addition to source targets, the basic components of a sputtering system are the enclosing vacuum chamber, pumps to maintain vacuum in the chamber, argon gas control, water cooling for targets and high voltage power supply.
FIG. 1D shows a bonded target-backing plate assembly in position for sputtering. The assembly fits into a circular opening in the wall of the sputter process vacuum chamber and essentially becomes part of the vacuum chamber wall, with the target (11) front surface in vacuum. The internally threaded boss protruding from the center of the reverse side of backing plate (12) serves to engage bolt (110) for clamping cover plate (14) to the backing plate. The circular ring boss on the reverse side of backing plate (12) provides a seat for elastomer O-rings seals for cooling water and also vacuum. For clarity, only vacuum seal O-ring (18), compressed between insulator (15) and chamber wall (19), is labeled.
Cover plate (14) is drawn tightly against the rear side of backing plate (12) with the aid of threaded bolt/nut (110). A space between cover plate (14) and the rear of backing plate (12) defines a channel for flow of cooling water which enters the gap through fitting (16) and exits through a similar fitting, not shown. A circular insulating ceramic ring (15) is positioned at the outer portion of the rear of backing plate (12), interposed between the backing plate and cover plate (14) and provides electrical isolation between the wall of the vacuum chamber (19) and the target-backing plate-cover plate assembly. High voltage connection to the target assembly is made through terminal (17).